Analog to digital converters with oversampling

ABSTRACT

Embodiments of the invention include an oversampling Analog to Digital Converter that uses uneven non-overlapping clock phases to reduce switched capacitor circuit power consumption. A return-to-zero sub phase of one of the clock phases may also be used for feedback reference capacitors. A delay lock loop may be combined with the non-overlapping clock phase generator to control accurate timing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation of co-pendingU.S. Non-Provisional application Ser. No. 15/007,054, entitled ANALOG TODIGITAL CONVERTERS WITH OVERSAMPLING, filed Jan. 26, 2016, which is acontinuation of U.S. Non-Provisional application Ser. No. 14/605,952,entitled ANALOG TO DIGITAL CONVERTERS WITH OVERSAMPLING, filed Jan. 26,2015, which in turn claims benefit from U.S. Provisional Application No.61/931,525, entitled ANALOG TO DIGITAL CONVERTERS WITH OVERSAMPLING,filed on Jan. 24, 2014, the contents of all of which are incorporated byreference herein.

FIELD OF THE INVENTION

This disclosure is directed to Analog-to-Digital Converters (ADCs) and,more specifically, to ADCs that consume relatively low amounts ofelectrical power.

BACKGROUND

Low power circuit design is one of the most active IC design arenas inmodern electronics, at least in part because of the constant pressure toincrease battery life in mobile devices. As oversampling analog todigital converters (ΔΣ ADCs) have wide applications in modernelectronics, reducing their power consumptions while maintainingperformance has significant implications in new product development.

Unlike Nyquist rate ADCs, where analog signal is digitized by a set ofquantized reference levels of which the number of quantization levelsdirectly determines the resolution of an ADC, oversampling ADCs use onlya small number of quantization levels to achieve high conversionresolution. For example, a ΔΣ ADC with a 1-bit quantizer can achieveconversion resolutions of more than 12-bit. The large amount ofquantization noise in a ΔΣ ADC is pushed outside the signal band by aloop filter and only a small amount of in-band quantization noiseremains. The subsequent digital filter can remove the out-of-bandquantization noise to achieve high resolution. In other words, theanalog difficulties of designing high precision reference levels aretraded with digital signal processing, which is much easier toimplement. The architecture of a typical low pass loop filter ΔΣ ADC isshown in FIG. 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a conventional ADC.

FIG. 2 is a graph of frequency responses of two known loop filters (toppanel) and their corresponding noise transfer functions (NTFs, bottompanel).

FIG. 3 is a functional block diagram of a conventional ADC modulatorincluding a loop filter.

FIG. 4 is a circuit diagram of a conventional switched capacitorimplementation of the loop filter of FIG. 3.

FIG. 5 is graph of an ideal loop filter frequency response of a 4thorder 1.5-bit ADC.

FIG. 6 is a graph of a degraded frequency response of the loop filterhaving a conventional passive summer.

FIG. 7 is a circuit diagram of a loop filter circuit including ade-coupling technique according to embodiments of the invention.

FIG. 8 is a graph illustrating a frequency response of the optimizedloop filter of FIG. 7.

FIG. 9 is a timing chart illustrating uneven non-overlapping clockphases used in embodiments of the invention.

FIG. 10 is an example circuit diagram that may be used to implement areturn-to-zero function according to embodiments of the invention.

FIG. 11 is an example circuit diagram of a delay lock loop that may beused in embodiments of the invention to accurately control timing.

DETAILED DESCRIPTION

An optimize quantization noise transfer function (NTF) may be used tominimize ΔΣ ADC power consumption.

The resolution of an Analog to Digital Converter (ADC) is measured byits Signal to Noise Ratio (SNR). Given the maximum signal level that anADC can handle without excessive distortion, a lower noise level leadsto higher SNR. Like any other type of ADCs, noise of a ΔΣ ADC has twocomponents: quantization noise and circuit noise. While circuit noisecan be reduced by using more power and occupying larger chip area,signal band quantization noise can be reduced by a proper quantizationnoise transfer function (NTF), which is determined by the loop filtershown in FIG. 1. It is desirable to have an NTF that can reduce in-bandquantization noise to a much lower level, so that the total in-band ADCnoise is dominated by circuit noise only.

Reducing in-band quantization noise will increase out-of-bandquantization noise, since a total quantization noise power is constant.The maximum value of the NTF is defined as noise gain, which is ameasure of out-of-band quantization noise. Empirical study shows thatnoise gain has a direct impact on loop stability: the larger the noisegain, the smaller the maximum signal level a ΔΣ ADC can handle before itbecomes unstable or overloaded. This is where the design art comes in:on one hand it is desirable to diminish in-band quantization noise andtherefore maximize noise margin for the circuit to reduce power; on theother hand it is undesirable to have too much NTF noise gain, otherwisethe maximum signal level has to be reduced for loop stability, which inturn will demand more power to reduce circuit noise to achieve thetarget SNR. The art is to find the optimized NTF for minimum powerconsumption. The following optimization data is disclosed from an actual100 dB audio band ADC development.

FIG. 2 shows the frequency responses of two loop filters (top panel) andtheir corresponding noise transfer functions (NTFs, bottom panel). Thesolid curves lead to lower in-band quantization noise but higher NTFnoise gain (1.7), and the dashed curves result in higher in-bandquantization noise but lower NTF noise gain (1.4).

In the development of a 100 dB audio band (20 -20 kHz) ΔΣ ADC withsample rate of 6 MHz and internal 1.5 -bit quantizer, the above two NTFsare compared in terms of power consumption. The NTF1 with noise gain of1.72 (solid curve) gives −149 dBFS (decibels relative to full scale)audio band quantization noise, and the NTF2 with noise gain of 1.40(dashed curve) gives -125 dBFS audio band quantization noise. For aSNR=100 dB, a −I 15 dBFS quantization noise only degrades SNR by 0.1 dB,therefore both NTF1 and NTF2 have little impact on total ADC noisefloor. However, because NTF1 has larger noise gain than NTF2 does, amaximum signal level of an ADC with NTF1 will be 20 % lower than that ofan ADC with NTF2 (K. C. H. Chao, S. Nadeem, W. L. Lee and C. G. Sodini,“A Higher Order Topology for Interpolative Modulators for OversamplingA/D Converters”, IEEE Trans. Circuits and Sys., vol. CAS-37, pp.309-318, March 1990 ). To achieve the same SNR of 100 dB, the circuit noiseof an ADC with NTF1 needs to be 20 % lower than that of ADC with NTF2.Since reduction of noise by 20 % increases power by 44 %, a loop filterwith NTF2 was used in silicon. Lab measurement showed that both SNR andpower consumption match design targets.

Note that further reduction of noise gain from NTF2 is not suggested,because a design margin should be left for in-band quantization noisefor process variations. In this design example, a 10 dB design margin isused since NTF2 has −125 dBFS in-band quantization, while −115 dBFSin-band quantization noise will degrade an ADC SNR by 1 dB. In otherwords, an optimized NTF in this type of ΔΣ ADC has noise gain of 1.4.

In summary, to realize a low power ΔΣ ADC design, the first step is tooptimize its loop filter for optimized NTF. An optimized loop filtershould be one that maximizes signal swing while keeping the in-bandquantization noise barely below the significant level at which it startsto have impact on total noise.

In practical design, however, it is not trivial to design a loop filterthat is faithful to its optimized target over all process, supplyvoltage, and temperature variations. As a common practice, in-bandquantization noise is often suppressed excessively to ensure that it isalways below the significant level, which ends up with a non-optimizedloop filter that has power consumption higher than it otherwise couldbe.

A technique disclosed below addresses one common design issue thatdegrades loop filter accuracy.

A de-coupling technique is used to reduce coupling among integratorstages through a passive summer.

FIG. 3 is a block diagram that shows the architecture of a 4th order ΔΣADC used to develop the 100 dB audio band ΔΣ ADC mentioned above. Itsloop filter includes of 4 integrators and a summer. The last 2integrators form a resonator through a local feedback path. Globalfeedback of ADC output Y is outside a loop filter, as illustrated inFIG. 1, however, for convenience of circuit implementation, the networkthat performs subtraction between input X and feedback Y is normallyincluded within the loop filter, as FIG. 3 illustrates.

In low power design, a summer is commonly implemented as a passivenetwork instead of an active component. A conventional switchedcapacitor implementation of the loop filter of FIG. 3 is illustrated inFIG. 4.

FIG. 4 is a conventional switched capacitor implementation of the loopfilter architecture illustrated in FIG. 3.

The loop filter of FIG. 4 is driven by 2 non-overlapping clock phases:charging phase (φ1, top panel) and dumping phase (φ2, bottom panel). Inthe charging phase φ1, an input capacitor of each integrator is chargedby the output of its preceding stage. The input capacitor of integrator1 is charged by the input signal of ADC. At the same time, feedbackreference capacitors (Cp1 r and Cn1 r) are charged to the correspondingreference voltages, according to ADC output Y (Y=−1 as an example inFIG. 4) of previous cycle. The passive summer in FIG. 4 is created bycoefficient capacitors Cp1 s-Cp4 s and Cn1 s-Cn4 s , and summerreference capacitors Cps and Cns.

During charging phase φ1, all of the coefficient capacitors aredischarged to 0 and the summer reference capacitors are pre-charged tothe reference voltage. The summer output (LFout) is also discharged to0. All of the integrator stages are in hold state and isolated from eachother. In dumping phase φ2, a charge on input capacitor of each stage isdumped into an integrating capacitor of the corresponding stage. At thesame time, integrator outputs of all the stages are summed togetherthrough the capacitor network.

After a transient is settled near the end of dumping phase φ2, aquantizer makes a decision and a new ADC sample is latched for the nextcycle. Note that there are two complementary comparator configurationsin a 1.5-bit quantizer. In order to draw a legible illustration, oneconfiguration is omitted in FIG. 4.

If the amplifiers of integrators are ideal, with infinite gain andbandwidth, at the end of dumping phase φ2, the current state of eachintegrator should only depend on its previous state, and new chargesdumped from its input capacitor in the current cycle. In other words,the ideal state of each integrator (except integrator 3 and 4 that forma resonator through a local positive feedback path) should not depend onthe states of other integrators at the current cycle. In reality,especially in low power design, however, amplifiers always have finitegain and bandwidth, and the coupling among all integrators through thepassive summer network during dumping phase φ2 makes the settlingdynamics of each stage interfere with each other. As a result, a finalvoltage of each integrator will deviate from its ideal value.

FIG. 5 illustrates an ideal loop filter frequency response of a 4thorder 1.5-bit ΔΣ ADC, while FIG. 6 illustrates degraded frequencyresponse of the loop filter with a conventional passive summer.

FIG. 5 shows the target frequency response of an optimized loop filterin terms of power consumption. If it is implemented with a conventionalpassive summer as illustrated in FIG. 4, the frequency response islargely deviated from its target as shown in FIG. 6. (1) no stagebehaves as an integrator below 10 kHz; (2) gains are severely reducedfor all stages below resonate frequency; (3) gains of integrator 1 andintegrator 2 are perturbed by resonator at resonating frequency; (4)unit gain frequency of loop filter output is reduced from target 550 kHzto about 473 kHz. Such a degradation will increase in-band quantizationnoise substantially, e.g., 35 dB at 1 kHz and 22 dB in audio band. Theoptimized NTF2 mentioned above that gives audio band quantization noiseof −125 dBFS, if implemented with conventional passive summer as shownin FIG. 4, will end up with −103 dBFS audio band quantization noise,which is only 3 dB lower than the specified total noise of −100 dBFS. Inorder to achieve resolution of SNR-100 dB, circuit noise needs to bebelow −103 dBFS, or power consumption has to be doubled. Therefore, anon-optimized NTF, such as NTF1 mentioned above, needs to be used tocompensate for this loop filter degradation. But, it also increasespower consumption for the other reason as explained above.

A de-coupling technique is disclosed below that can improve the loopfilter accuracy significantly. Since integrator 3 and 4 are coupled toeach other by a resonator loop any way, the decoupling technique focuseson isolating integrator 1 and integrator 2 only. The additional couplingbetween integrator 3 and 4 through the passive summer will alter overallloop filter response slightly, however, due to large gains fromintegrator 1 and 2, the input referred error will be negligible.

FIG. 7 is a circuit diagram of a switched capacitor implementationaccording to embodiments of the invention. One of the differencesbetween the circuit illustrated in FIG. 7 and the one illustrated inFIG. 4 is that, in charging phase φ1, coefficient capacitors ofintegrator 1 and integrator 2 are charged by integrator outputs insteadof discharged to 0. Note that integrator I and integrator 2 aredecoupled from each other in this phase because of the low impedance ofanalog ground. In dumping phase φ2, coefficient capacitors of integrator1 and integrator 2 carry the charge to the passive summer whileintegrator 1 and integrator 2 are de-coupled from the transient of otherstages. Note also that, because the charges on coefficient capacitors ofintegrator 1 and integrator 2 do not include feedback of ADC output Y ofprevious cycle, a feedback reference network is implemented inside thepassive summer by capacitors Cpsr and Cnsr as shown at the right side ofFIG. 7. With proper sizes of coefficient capacitors, the circuit of FIG.7 can be mathematically identical to the circuit of FIG. 4, but theimplementation accuracy will be much higher, as shown in FIG. 8.

FIG. 8 illustrates a frequency response of the loop filter that uses thedecoupling technique shown in FIG. 7 and according to embodiments of theinvention.

Compared to that of FIG. 6, the frequency response of the loop filter ofFIG. 7 is much closer to its mathematical target shown by FIG. 5. Thegain reduction at frequencies below 100 Hz is due to finite gain ofamplifiers, which will have only negligible impact on quantization noiseshaping.

The de-coupling technique described above leads to an accurate circuitimplementation of a loop filter. Because design margin can be reducedsubstantially, power consumption is also be minimized with an optimizedloop filter.

In conventional switched capacitor circuit design, charging phase φ1 anddumping phase φ2 have equal duration, as illustrated in FIGS. 4 and 7.Careful study of the circuit configuration reveals that powerconsumption can be further reduced by using uneven, non-overlappingclock phases.

In charging phase φ1, all amplifiers are in holding state, thereforethey are much quieter compared to their dynamics during dumping phaseφ2, where new charges are being integrated. This is especially true forintegrator 1, due to a large step size of the quantized feedback signal.As is common in switched capacitor circuit design, more than half of thepower is consumed by the amplifier of integrator 1 to increase itsbandwidth and slew rate in order to settle with high accuracy duringdumping phase φ2.

Based on above observation, the amplifier bandwidth and slew rate can bereduced by increasing the dumping phase φ2 duration. To keep the samesample rate, a duration of the charging phase φ1 is reduced accordingly.If ¾ of a cycle is allocated to dumping phase φ2, and ¼ allocated tocharging phase φ1, as shown in FIG. 9, the power consumptions of allintegrators can be reduced by ⅓, as long as settling accuracy ofcharging phase φ1 is not compromised, which is usually the case becausethe loadings of amplifiers in charging phase φ1 are much lightercompared to that in dumping phase φ2.

FIG. 9 illustrates uneven non-overlapping clock phases to reduceswitched capacitor circuit power consumption. Reduction of chargingphase φ1 requires careful timing control. The feedback referencecapacitors Cp1 r and Cn1 r in FIG. 7 are charged in phase φ1 of currentcycle to a new reference voltage corresponding to ADC output Y at theprevious cycle. To reduce inter-symbol-interference (ISI) that causesdistortion, these two capacitors are always discharged to 0 before theyare charged to the new reference voltage. In other words, there is areturn-to-zero sub phase inside charging phase φ1 for feedback referencecapacitors, which is illustrated in FIG. 9 as rtz, and the circuitimplementation is illustrated in FIG. 10. The actual charging time ofthe feedback reference capacitors will be the remaining duration ofcharging phase φ1 after the return-to-zero sub phase ends.

The return-to-zero sub phase should not be too short, otherwise the ISIelimination is insufficient. The rtz sub phase also should not be toolong, otherwise the feedback reference capacitors cannot accuratelysettle to the new reference voltage before phase φ1 ends. Settlingaccuracy is exponentially degraded with settling time reduction. Forexample, a −100 dB settling accuracy requires at least 11.5 τ, where τis the settling time constant. If total duration of φ1 is 40 ns, and 10ns is allocated to return-to-zero phase, then only 30 ns is left forfeedback reference capacitors to settle, which requires a settling timeconstant τ=2.6 ns. If the timing of the return-to-zero phase is notcarefully controlled, IC fabrication process variations and circuittemperature drift can easily change the return-to-zero phase by 2.6 nssimply through rising/falling edge and propagation delay variations, andthe feedback reference cap settling time can be reduced from 11.5 τ to10.5 τ. As a result, in such a case, the settling accuracy is reducedfrom −100 dB to −91 dB.

To accurately control the timing of return-to-zero phase, a delay lockloop (DLL) can be combined with the non-overlapping clock phasegenerator, as shown in FIG. 11.

In the example illustrated in FIG. 11, a delay line inside the DLLdelays its input φ1 pulse rising/falling edges by the entire duration ofphase φ1, regardless of fabrication process, supply voltage and circuittemperature variations. A reset signal of rtz is tapped off at the ¼ ofthe delay line. With such a design, rtz phase accurately starts at thebeginning of phase φ1, and ends at ¼ duration of phase φ1.

The power consumption of the DLL illustrated in FIG. 11 can be less than15 μA. Compared to current savings from integrator amplifiers byintroducing uneven non-overlapping clock phases, this overhead isnegligible.

Embodiments of the invention include circuit elements to realize anoversampling analog-to-digital converter with minimum power consumption.More specifically, the audio ADC described above has better performanceand lower power than any ADC available previously. The optimizationmethod by selecting proper noise transfer function to minimize powerconsumption is general to all oversampling analog to digital converters.Although the de-coupling technique is illustrated by a switchedcapacitor implementation of the loop filter, it can be applied to otherimplementations using a passive summer. The uneven non-overlap clockphase technique is unique to switched capacitor implementation, however,as a general power saving technique, it can also be applied to otherswitched capacitor circuits beyond oversampling analog to digitalconverters, such as switched capacitor filters, switched capacitoramplifiers, etc. Among the benefits are: (1) Optimization of noisetransfer function for power efficiency; and (2) de-coupling techniquethat ensures accurate loop filter frequency response therefore reducedesign margin, which makes the optimization in (1) particularlymeaningful.

Although specific embodiments of the invention have been illustrated anddescribed for purposes if illustration, it will be understood thatvarious modifications may be made without departing from the spirit andscope of the invention.

What is claimed is:
 1. An analog-to-digital modulator, comprising: aswitched-capacitor loop filter, including: integrators operable in acharging phase and a dumping phase, in which at least some of theintegrators are decoupled from the remaining integrators during thedumping phase.
 2. The analog-to-digital modulator according to claim 1,in which the charging phase is driven by a first clock and the dumpingphase is driven by a second clock, and in which the first and secondclocks have non-overlapping clock phases.